Occasions arise when devices are manufactured that contain surfaces whose shapes must be accurately controlled to maintain a necessary level of optical performance. Optical MEMS devices are one such device. For example, in an optical cross-connect design, a reflecting mirror must stay flat to suppress any disturbance to the focusing/collimating action of the lens system. Typically, the material which is utilized to fabricate the structural portion of the optical surface does not possess the required optical properties, and thus coatings generally must be applied to the optical surface. The coatings are frequently stressed, which can cause the optical surface to deform, resulting in a loss of performance.
To inhibit this effect, it is desirable to make the structural portion of the optical surface as stiff as possible. Usually, greater stiffness is achieved by making the structural portion thicker. However, making the structural portion thicker leads to disadvantages when such a device is employed. FIG. 1 illustrates the dimensions (length, width, thickness) of a generic spring.
In known optical MEMS devices, the same layer of material used to form the optical surface is also used to fabricate a flexure structure. The flexure structure is generally utilized to connect an optical device, such as a mirror, with a support structure. For example, FIG. 2 illustrates a known MEMS device 10 including a flexure structure 12 formed from the same layer of material used to form the optical surface 14 of an optical device 16.
A flexure structure, such as flexure structure 12, allows the optical layer 14 of the optical device 16 to rotate in a direction A when the MEMS device 10 is actuated. Generally, electrostatic force is used to actuate MEMS devices. It is desired to fabricate the MEMS device 10 in such a way as to limit the amount of electrostatic force needed to actuate the device. The amount of actuating force necessary is that which can overcome the stiffness of the flexure structure 12. Thus, it is known to make the flexure structure 12 relatively compliant. It is further known that a large degree of control over the compliance of the flexure structure 12 is needed to optimize the MEMS device 10 design.
Highly compliant flexure structures can be fabricated by reducing at least one dimension of the flexure structure. For example, in the instance where the flexure structure is fabricated from the same layer of material as the optical structure, such as the flexure structure 12, the only dimension which is reducible is the width. Making narrow but deep, i.e., high aspect ratio, structures, however, presents a processing challenge and tends to put a constraint on the thickness T (FIG. 1) of the optical layer 14. Making the flexure structure 12 more compliant by extending its length L (FIG. 1) encounters other problems, such as requiring a great amount of space and could lead to undesirable deflection modes.
Thus, the design requirements for the optical surfaces of known optical devices, which should be made as stiff as possible, are in conflict with those for flexure structures, which should be made as compliant as possible.
As shown in FIG. 3, another MEMS device 110 is shown including a flexure structure 112 out of plane with an optical surface 114 of an optical device 116. The flexure structure 112 is out of plane with the optical surface 114 by virtue of being mounted on posts 118. Further, such a design, as described in U.S. Pat. No. 6,201,629 (McClelland et al.), is one in which the flexure structure 112, and not the optical device 116, is configured to be actuated.
Conventionally, one way of fabricating a MEMS device involved fabricating the flexible layer and the mirror on one chip and the driver electronics on another chip and flip chip bonding the two chips together. The use of flip chip bonding has disadvantages. For example, alignment is not as accurate as fabricating the MEMS device from one wafer. Further, flip chip bonding adds an extra complicated, and hence expensive, step to the fabrication process which adds to fabrication costs and often leads to decreases in yield.
There exists a need for devices having a flexure structure whose dimensions can be decoupled from the dimensions of other components of the optical device. There further exists a need for a MEMS optical device which is fabricated from two different materials planarly aligned and which does not require complicated flip chip bonding.